EP2396744B1

EP2396744B1 - High-resolution optical code imaging using a color imager

Info

Publication number: EP2396744B1
Application number: EP10741731.3A
Authority: EP
Inventors: Bryan L. Olmstead; Alan Shearin
Original assignee: Datalogic ADC Inc
Current assignee: Datalogic ADC Inc
Priority date: 2009-02-11
Filing date: 2010-02-11
Publication date: 2016-06-01
Anticipated expiration: 2030-02-11
Also published as: EP2396744A2; WO2010093806A3; US8118226B2; EP2396744A4; CN102317951B; WO2010093806A2; CN102317951A; US20100200658A1

Description

Technical Field

The field of this disclosure relates generally to systems and methods of data reading, and more particularly but not exclusively to reading of optical codes (e.g., bar codes).

Background Information

Optical codes encode useful, optically-readable information about the items to which they are attached or otherwise associated. Perhaps the most common example of an optical code is the bar code. Bar codes are ubiquitously found on or associated with objects of various types, such as the packaging of retail, wholesale, and inventory goods; retail product presentation fixtures (e.g., shelves); goods undergoing manufacturing; personal or company assets; and documents. By encoding information, a bar code typically serves as an identifier of an object, whether the identification be to a class of objects (e.g., containers of milk) or a unique item. A typical linear or one-dimensional bar code, such as a UPC code, consist of alternating bars (i.e., relatively dark areas) and spaces (i.e., relatively light areas). The pattern of alternating bars and spaces and the widths of those bars and spaces represent a string of binary ones and zeros, wherein the width of any particular bar or space is an integer multiple of a specified minimum width, which is called a "module" or "unit." Thus, to decode the information, a bar code reader must be able to reliably discern the pattern of bars and spaces, such as by determining the locations of edges demarking adjacent bars and spaces from one another, across the entire length of the bar code.
Bar codes are just one example of the many types of optical codes in use today. Higher-dimensional optical codes, such as, two-dimensional matrix codes (e.g., MaxiCode) or stacked codes (e.g., PDF 417), which are also sometimes referred to as "bar codes," are also used for various purposes.
Different methods and types of optical code reading devices are available for capturing an optical code and for decoding the information represented by the optical code. For example, image-based readers are available that include imagers, such as charge coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) imagers, that generate electronic image data that represent an image of a captured optical code. Such a device is known from WO 2005/072 193 A2 . Image-based readers are used for reading one-dimensional optical codes and higher-dimensional optical codes. Because optical codes most often include dark and light patterns (e.g., black and white) that represent binary data, imagers of image-based readers are typically monochrome so that uniform sensitivity for each pixel of the imager is achieved. Also, typical image-based readers include light sources that illuminate the image-based reader's field of view with narrowband visible light to achieve high optical resolution by avoiding chromatic aberration and polychromatic diffraction effects. Narrowband light sources typically used for imaging include laser diodes, having a bandwidth on the order of 5 nanometers (nm), and light emitting diodes (LEDs), having a bandwidth on the order of 50 nm.
Common imagers made for image capturing devices, such as still cameras and video cameras, however, are color imagers - not monochrome. Because imagers made for many image capturing devices are color, color imagers are generally made in higher volume and have become less expensive than monochrome imagers. Some image-based readers have included color imagers, but the present inventors have recognized that those readers do not effectively achieve high optical resolution comparable to monochrome image-based readers with the same number and size of pixels.

Summary of the Disclosure

This disclosure describes improved optical reading devices and associated methods. In accordance with the invention, there is provided the method of claim 1 and the device of claim 8.
One embodiment is directed to an optical code reading device that includes a color image sensor array positioned to sense light reflected from an object in a field of view of the optical code reading device and to produce from the sensed reflected light image data representing an image of the object. The color image sensor array has a first set of sensor elements that are sensitive to a first visible wavelength band of light, and a second set of sensor elements that are sensitive to a second visible wavelength band of light. The first and second sets of sensor elements are also sensitive to light within an infrared wavelength band. The optical code reading device includes an artificial illumination source positioned to illuminate the field of view of the optical code reading device with light that is incident upon and reflected from the object in the field of view toward the image sensor array. The illumination source is operable to produce infrared light having wavelengths within the infrared wavelength band so that, upon illumination of the field of view, at least some sensor elements of each of the first and second sets are sensitive to the infrared light and contribute to production of the image data.
Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a diagram of an imaging system according to a preferred embodiment.
Figure 2 is a diagram of a color filter pattern of a color image sensor array of the imaging system of Figure 1.
Figure 3 is a graph of the sensitivity of blue, green, and red sensor elements as a function of light wavelength of an illustrative color image sensor array used in the imaging system of Figure 1.
Figure 4 is a flowchart showing the operational steps of the imaging system of Figure 1.
Figure 5 is a flowchart of an illumination calibration method according to one embodiment.

Detailed Description of Preferred Embodiments

With reference to the above-listed drawings, this section describes particular embodiments and their detailed construction and operation.
Figure 1 is a diagram of an imaging device 100, such as an optical code reading device, according to one embodiment. The imaging device 100 includes a color image sensor array 102 contained within a housing 103 of the imaging device 100. The color image sensor array 102 includes a first set of sensor elements 104 and a second set of sensor elements 106. The sensor elements of the color image sensor array 102 may be arranged in a one-dimensional array or, preferably, a two-dimensional array. The color image sensor array 102 may be a CCD, such as a frame-transfer or interline-transfer CCD. The color image sensor array 102 may alternatively be a CMOS imager, such as a global shuttered or rolling-reset CMOS imager. Suitable imagers or image sensor arrays are available, for example, from Aptina Imaging Corporation of San Jose, California, USA including, but not limited to, a model MT9V022 VGA color imager. Imagers are available from many manufacturers and are available in various resolutions (numbers of pixels). For higher resolution applications, other imagers from Aptina Imaging Corporation are suitable, including the model MT9M001 with 1.3 megapixels, the model MT9M002 with 1.6 megapixels, and the model MT9P001 with 5 megapixels. As imaging technology advances, imager resolution increases and other imagers may also be suitable.
The color image sensor array 102 may include more than two sets of sensor elements. For example, the color image sensor array 102 may include three sets of sensor elements 104, 106, and 202 arranged in a Bayer pattern as shown in Figure 2. Each set of sensor elements corresponds to a different color. For example, the first set 104 may be sensitive to light having wavelengths that correspond to the color green (G) (wavelengths ranging between about 500 nanometers (nm) and about 600 nm), the second set 106 may be sensitive to light having wavelengths that correspond to the color red (R) (wavelengths ranging between about 600 nm and about 750 nm), and the third set 202 may be sensitive to light having wavelengths that correspond to the color blue (B) (wavelengths ranging between about 400 nm and about 500 nm). Moreover, a color filter associated with each sensor element of the different sets appreciably filters out visible light that does not correspond to its color (i.e., the color filters associated with the first set 104 appreciably block out red and blue light).
Figure 3 is a graph depicting an example of the quantum efficiency percentage versus the wavelength of light incident upon red, green and blue sensor elements of the model MT9V022 VGA color imager available from Aptina Imaging Corporation that may be used as the color image sensor array 102. A curve 104', corresponding to the spectral sensitivity of the sensor elements of the first set 104, has a local peak 104a' at a wavelength corresponding to the color green. A curve 106', corresponding to the spectral sensitivity of the sensor elements of the second set 106, has a local peak 106a' at a wavelength corresponding to the color red. A curve 202', corresponding to the spectral sensitivity of the sensor elements of the third set 202, has a local peak 202a' at a wavelength corresponding to the color blue. The curves 104', 106', and 202' also have respective local peaks 104b', 106b', and 202b' near a common wavelength that corresponds non-visible light - in this case infrared light. In other words, the first set 104, the second set 106, and the third set 202 are not only sensitive to green, red, and blue light, respectively, but also to light within an infrared wavelength band. The infrared wavelength band, to which the first set 104, the second set 106, and the third set 202 are sensitive, may be relatively narrow (e.g., no more than about 100 nm) and may include 850 nm. Also, the quantum efficiency percentage associated with the local peaks 104b', 106b', and 202b' may be substantially the same or within a narrow percentage range. In other words, the sensitivity of the sets 104, 106, and 202 to infrared light may be substantially equal or within a narrow sensitivity range (e.g., about five percent in quantum efficiency) so that an average intensity value of light sensed by the first set 104 may be substantially equal to an average intensity value of light sensed by the second set 106 and an average intensity value of light sensed by the third set 202.
The color image sensor array 102 need not be limited to three sets of sensor elements or the colors red, green, and blue, and the color image sensor array 102 may include color filter patterns other than the Bayer pattern. For example, the color image sensor array 102 may include a cyan, yellow, green, and magenta (CYGM) filter or a red, green, blue, and emerald (RGBE) filter in which the sensor elements of the different colors are also sensitive to light within an infrared wavelength band. The color filter pattern used on the color filter array 102 may be chosen to achieve accurate color rendition or to improve sensitivity in a color photograph application. While these distinctions are not necessary in the present embodiment, the imaging device 100 and its associated methods are flexible to compensate for the effects of these various filters.
The imaging device 100 may also include one or more artificial illumination sources 108 (two illumination sources are depicted in Figure 1). The artificial illumination sources 108 may be mounted to a printed circuit board 110 upon which the color image sensor array 102 is also mounted. In a first embodiment, the artificial illumination sources 108 are operable to emit infrared illumination. The infrared illumination emitted by the artificial illumination sources 108 may be narrowband infrared illumination (e.g., illumination having a bandwidth less than about 100 nm). Also, the wavelength bandwidth of light emitted by the artificial illumination sources 108 preferably includes 850 nm, when using a color image sensor array with characteristics shown in Figure 3.
The imaging device 100 typically includes a suitable optical system 112 positioned to focus light upon the color image sensor array 102. The optical system 112 may include conventional optical components, such as one or more lenses, an aperture, and, in some cases, a mechanical shutter. As an alternative to a mechanical shutter, the color image sensor array 102 may include electronic shuttering means. The optical system 112 may also include one or more optical filters to block out certain wavelengths of light. In one example, when infrared illumination sources are selected for the artificial illumination sources 108, the optical system 112 excludes an infrared filter, which is operable to block out infrared light, and may include one or more optical filters that are operable to block out light that does not have wavelengths within the infrared wavelength band. Although the artificial illumination sources 108 are shown as being mounted on the printed circuit board 110, the artificial illumination sources 108 may be positioned in other convenient locations to provide illumination of the object 114.
A preferred operation of the imaging device 100 will now be described with reference to a flowchart 400 of Figure 4. The artificial illumination sources 108 illuminate the field of view 116 with infrared illumination (step 402) or, in an example not as claimed, illumination of another non-visible frequency at which all of the sensor elements have an acceptable response. If an object 114 (e.g., an optical code) is within the field of view 116 of the imaging device, infrared light reflects off the object 114 toward the optical system 112. Infrared light that is incident on the optical system 112 is focused by the optical system 112 onto the sensor elements of the color image sensor array 102 (step 404). Sensor elements of the first set 104, the second set 106, and the third set 202 sense the focused infrared light ( steps 406a, 406b, and 406c). The color image sensor array 102 produces image data based upon the infrared light that is incident on, and sensed by, the sensor elements of the first, second, and third sets 104, 106, and 202 (step 408). An enclosure 118 may cover the color image sensor array 102 except where the optical system 112 is located, so that an appreciable amount of light from sources other than the artificial illumination sources 108 does not reach the color image sensor array 102. Because each of the sets 104, 106, and 202 are sensitive to infrared light, each of the sets 104, 106, and 202 contribute to production of the image data and high-resolution infrared imaging of the object 114 may be achieved. Each of the sets 104, 106, and 202 may contribute to the production of the image data to a sufficiently equal extent that no one of the set 104, 106, or 202 contributes to the image data appreciably more than the other two sets. The resolution of an infrared image represented by the image data may be substantially equal to a resolution produced by a monochrome image sensor array having the same size of sensor elements and the same number of sensor elements as the sum of the number of sensor elements in the sets 104, 106, and 202. In other words, when illuminated with infrared light, the color image sensor array 102 may achieve a resolution substantially equivalent to a monochrome imager.
Though the present invention has been set forth in the form of its preferred embodiments, it is nevertheless intended that modifications to the disclosed systems and methods may be made without departing from inventive concepts set forth herein.

Claims

A method of imaging an object in a field of view of an optical code reading device, the optical code reading device having a color image sensor array, the method comprising:
illuminating a field of view of the optical code reading device with artificial infrared light that is incident upon the object, thereby producing reflected infrared light, the reflected infrared light having wavelengths within an infrared wavelength band;

sensing the reflected infrared light with a first set of sensor elements of the color image sensor array, the first set of sensor elements being sensitive to the infrared light and to visible light having wavelengths within a first visible wavelength band;

sensing the reflected infrared light with a second set of sensor elements of the color image sensor array, the second set of sensor elements being sensitive to the infrared light and to visible light having wavelengths within a second visible wavelength band different from the first visible wavelength band; and

producing image data from the first and second sets of sensor elements, the image data being derived from the infrared light sensed by the first and second sets of sensor elements and representing a high-resolution infrared image of the object.
A method according to claim 1, further comprising:
sensing the reflected infrared light with a third set of sensor elements of the color image sensor array, the third set of sensor elements being sensitive to the infrared light and to visible light having wavelengths within a third visible wavelength band different from each of the first and second visible wavelength bands; and

producing the image data from the first, second and third sets of sensor elements, the image data being derived from the infrared light sensed by the first, second and third sets of sensor elements.
A method according to claim 2, wherein the resolution of the infrared image represented by the image data is substantially equal to a resolution produced by a monochrome image sensor array having the same number of sensor elements as the sum of the numbers of sensor elements in the first, second and third sets of sensor elements.
A method according to claim 2 or 3, wherein the first, second and third visible wavelength bands correspond to the colors red, green and blue.
A method as in one of claims 1-4, wherein the infrared wavelength band includes 850 nm.
A method as in one of claims 1-5, wherein upon illumination of the field of view, sensor elements of each of the first and second sets of sensor elements are sensitive to the infrared light to a sufficiently equal extent such that an average intensity value of light sensed by the first set of sensor elements is substantially equal to an average intensity value of light sensed by the second set of sensor elements.
A method as in one of claims 1-6, further comprising:
intermittently blocking the reflected infrared light from reaching the sensor elements of the first and second sets by a mechanical shutter.
An optical code reading device (100) comprising:
a color image sensor array (102) positioned to sense light reflected from an object in a field of view of the optical code reading device and to produce from the sensed reflected light image data representing an infrared image of the object, the color image sensor array comprising:
a first set of sensor elements (104) that are sensitive to a first visible wavelength band of light, and

a second set of sensor elements (106) that are sensitive to a second visible wavelength band of light, different from the first visible wavelength band, wherein the first and second sets of sensor elements are sensitive to light within an infrared wavelength band; and

an artificial illumination source (108) positioned to illuminate the field of view of the optical code reading device with light that is incident upon and reflected from the object in the field of view toward the color image sensor array, the artificial illumination source being operable to produce infrared light having a wavelength within the infrared wavelength band so that, upon illumination of the field of view, at least some sensor elements of each of the first and second sets are sensitive to the infrared light and are adapted to contribute to production of the image data.
An optical code reading device according to claim 8, wherein the color image sensor array includes a third set of sensor elements that are sensitive to a third visible wavelength band different from each of the first and second visible wavelength bands of light and to light within the infrared wavelength bands
An optical code reading device according to claim 9, wherein the first, second and third visible wavelength bands correspond to the colors red, green and blue.
An optical code reading device as in one of claims 8-10, wherein the infrared wavelength band includes 850 nm.
An optical code reading device as in one of claims 8-11, wherein upon illumination of the field of view, sensor elements of each of the first and second sets of sensor elements are sensitive to the infrared light to a sufficiently equal extent such that an average intensity value of light sensed by the first set of sensor elements is substantially equal to an average intensity value of light sensed by the second set of sensor elements.
An optical code reading device as in one of claims 8-12, wherein the color image sensor is part of a rolling-reset CMOS imager.
An optical code reading device as in one of claims 8-13, further comprising:
a mechanical shutter positioned between the color image sensor array and the field of view.
An optical code reading device according to claim 9, wherein:
the first set of sensor elements are characterized by a first spectral sensitivity that varies as a first function of wavelength, the first function having a local peak at a first peak sensitivity wavelength and also having a local peak near an infrared wavelength;

the second set of sensor elements are characterized by a second spectral sensitivity that varies as a second function of wavelength, the second function having a local peak at a second peak sensitivity wavelength and also having a local peak near the infrared wavelength;

the third set of sensor elements are characterized by a third spectral sensitivity that varies as a third function of wavelength, the third function having a local peak at a third peak sensitivity wavelength and also having a local peak near the infrared wavelength;

the first, second and third peak sensitivity wavelengths are different wavelengths; and

the artificial illumination source is operable to produce radiation including light having the infrared wavelength.